The present invention relates to a pole position detector for a motor, employing magnetic sensors.
A conventional brushless DC motor is disclosed in Japanese Unexamined Patent Publication No. 11-215881 in which the motor has a driver employing magnetic sensors. The motor has a pole position detector for detecting a rotational position, or a phase angle, of the rotor The pole position detector has a disk and the rotation center thereof agrees with that of the rotor. The disk has magnetized peripheral areas whose number is the same as that of magnetic poles of the rotor. The pole position detector also has three positional signal generators (U, V, and W) along the periphery of the disk. The signal generators are spaced from each other at intervals of 60xc2x0 in mechanical angle (corresponding to 120xc2x0 in electrical angle when the rotor and disk have each four magnetic poles). The signal generators have Hall ICs, respectively, to generate rotational position signals CSU, CSV, and CSW.
Another pole position detector of related art directly detects magnetic flux from each magnet incorporated in a rotor of a motor. The detector arranges a Hall element a predetermined distance away from an end face of the rotor that crosses a rotor shaft. To collect leakage magnetic flux from the magnet, the Hall element has a magnetic piece on the back thereof.
Still another prior art Japanese Unexamined Patent Publication No. 9-121584 discloses a motor that employs magnetic sensors such as Hall elements to directly detect leakage flux from rotor magnets and determine a pole position.
However, the pole position detector of the prior art must employ the separate disk in addition to the rotor, to increase the size of the detector. Namely, the prior art increases in size, weight, and cost of a motor to which the magnetized/detecting is applied.
The pole position detector of the related art is vulnerable to stat or current. Namely, the output of the Hall element is phase-shifted by stat or current, to incorrectly detect a pole position. When stat or coils pass no current, the output of the Hall element correctly responds to spatial distribution of magnetic field originated from the magnet of the rotor which varies according to the rotational angle of the magnetic pole thereof along a circumference thereof. When the stat or coils pass current, the current forms additional magnetic flux that changes an apparent spatial distribution of magnetic flux, thereby shifting a rotation angle, fluctuating or decreasing torque, and deteriorating motor control.
FIGS. 1 and 2 schematically show shifts caused by stator current in pole positions to be detected by the Hall element. Leakage flux from the rotor changes in response to the position of the magnet as indicated with a curve (a) of FIG. 1. With a d-axis current Id=0 (phase angle xcex2=0), a q-axis current Iq is passed through the stator coils to produce magnetic flux. This magnetic flux is opposite to and 90xc2x0 (electrical angle) ahead of the magnetic flux (a) produced by the rotor, as indicated with a curve (b) of FIG. 1. The Hall element facing the end face of the rotor is influenced by the sum of the leakage flux from the rotor and the 90xc2x0-advanced magnetic flux from the stator coils, as indicated with a curve (b) of FIG. 2. A pole position detected by the Hall element at this time is a function of the magnitude of magnetic flux determined by the current and a phase angle. The detected pole position, therefore, involves a delay in phase from a true pole position. If a field-weakening operation is carried out on the motor under this state by increasing the d-axis current, the phase of magnetic flux produced by stator current advances as the current phase angle xcex2 increases, as indicated with curves (b) to (f) of FIG. 1. Consequently, the phase of magnetic flux (pole position) detected by the Hall element according to the sum of magnetic flux from the magnet and magnetic flux from the stator coils advances as indicated with curves (b) to (f) of FIG. 2. In this way, detecting a pole position based on leakage flux from rotor magnets with the use of magnetic sensors such as Hall elements is prone to fluctuations caused by stator current.
Japanese Patent Application No. 2000-033500 filed by this inventor has disclosed a technique of employing magnetic materials to collect rotor leakage flux at ends of the magnetic materials to improve the correctness of pole position detection. This technique is advantageous in reducing the influence of stat or current but is affected by the stat or current in each gap or occupied nonmagnetic material between the magnetic materials, to slightly change the phase of the output of each Hall element.
There are conventional devices such as resolvers and rotary encoders that precisely detect a pole position of a motor. These devices, however, are expensive compared to the detectors that employ magnetic sensors such as Hall elements, and therefore, are inapplicable to cost-sensitive applications.
An object of the present invention is to provide a low-cost pole position detector for a motor, employing magnetic sensors to correctly detect the rotational position of each magnetic pole of the motor which leads the desired rotational angle of the rotor thereof.
In order to accomplish the object, one aspect of the present invention provides a pole position detector for a motor, having a magnetic sensor for providing a time course of an electric signal representing spatial magnetic distribution of a rotor of the motor or rotational member linked with the rotor along a circumference thereof caused by rotation thereof, and a computation unit for detecting a base pole position of the rotor from the electric signal, detecting a stat or current magnitude and a current phase, correcting the detected base pole position according to the detected base pole position, stat or current magnitude, and current phase, and providing the corrected base pole position as a pole position.
According to another aspect of the present invention, a detecting method of pole position for a motor comprises detecting magnetic changes caused by rotation of a rotor of the motor or by rotation of a rotational member linked with the rotor, detecting a base pole position of the rotor according to the detected magnetic changes, and correcting the detected base pole position according to the detected base pole position, a stat or current magnitude, and a current phase.